This invention relates to the combined use of passive IR detectors and laser radars, ladars, or laser range finders, and more particularly to a method and apparatus for correcting boresight errors between the two systems which actively corrects boresighting errors.
Both and a ladar and a passive IR detector have been utilized in combination in order to identify the position of incoming missiles and associated decoys so as to be able to direct kill vehicles to intercept selected ones of these targets. Typically this dual mode detection system is employed to initially provide coarse detection of the position of incoming targets and decoys within a target cloud through the use of an IR detector. This is followed by the use of an active laser system to pinpoint each of the targets so that individual kill vehicles can be launched from an intercept missile to destroy selected targets.
The course spatial position of these targets is first determined by the infrared detection system such that the targets are determined to exist with accuracy limited to the accuracies of IR systems. The reported IR position is then refined through the use of ladar techniques. When these two systems are used together they must be co-boresighted and the accuracy of the co-boresighting is critical.
The intercept missile is guided by ground based radar information to a point where the IR system can acquire the objects (typically 500 to 1000 kilometers from the intercepting missile). Assuming that the closing velocity is 10 to 14 kilometers a second, then one has between 50 and 100 seconds for the whole engagement.
While detection may occur between 500 and 1,000 kilometers, actual targeting occurs by IR and laser avionics carried on the interceptor missile when the targets are typically between 300 to 500 kilometers out. At 10 kilometers per second this portion of the engagement scenario can take no longer than 30 to 50 seconds.
The infrared detection field of view is typically on the order of, 2 degrees which at for instance 300 kilometers provides a field of view having a diameter of about 10.5 kilometers. In a battlefield scenario, an incoming vehicle may be deploying decoys such that within this 10.5 km field of view there may be as many as 200 to 300 individual targets, one being the incoming threat vehicle itself and the others being decoys.
In order to provide more exact information about target threats it is common to provide a multimode system involving not only a passive IR detector, but also an active laser system. It is therefore important that the laser be coboresighted with the IR detector so that the laser beam can be directed directly on top of a target reported by the IR detector, with the ladar range and line of sight to the threat vehicle further defining the position of a target at the time of detection of a returned laser pulse. Once having ascertained its inertial position one can launch multiple kill vehicles from the missile to destroy the target.
Note that the laser beam is an exceedingly narrow beam that for long range operation would be on the order of 20 microradians. At 500 kilometers away this beam subtends an area having a diameter of about 10 meters. If the laser boresight were perfectly aligned to the infrared detector boresight, then the laser pointing apparatus would assure that a laser beam projected along the infrared detector boresight centered on the intended target would strike the target.
However, even when the boresights are aligned and checked at the factory, there are inherently biasing errors referring to a misalignment between the IR and laser boresights which are static meaning they are always in one direction, also called bias error. There are also errors having to do with measurements, and these are random errors. The result of these two errors is that the position of a target reported by the IR detector cannot be used for laser pointing because the laser beam will be offset from the line of sight indicated by the IR detector. Thus with a very narrow laser beam it is very likely that the laser beam will miss the target if one simply fires a laser beam along the IR boresight direction. The miss is a result of an erroneous assumption that the laser boresight is perfectly aligned with the IR detector boresight.
Because of these errors, in a tactical situation the ladar beam is made to execute a search pattern such that the beam is directed in a search pattern that dances around the assumptive location of the target, the assumptive location being that which is established by the IR detector.
For each target in the target cloud, the laser beam must execute the search pattern and when there is a return from a particular target, the laser beam""s direction is sufficiently exact to be used for directing one or more kill vehicles to the target.
However, if one considers 200 targets in a target cloud, and further assuming that each of the targets identified by the passive IR detector must be scanned, the overall scan time for the ladar is much too long. Thus during the 30 to 50 second engagement scenario all targets cannot be found in time.
What is therefore necessary is a real time correction system which can correct the pointing mirror for the ladar so as to eliminate bias errors and random errors. In so doing it will only take one laser probe to hit the intended target. This would eliminate time consuming scanning. One needs to correct bias and random boresight errors by using measured information as opposed to predicted information so as to minimize the scanning of the laser beam in order to actually illuminate the target identified by the infrared detector.
In order to minimize the scanning times required and thus be able to illuminate incoming targets quickly enough to assure destruction, in the subject invention an active boresight error correction system is employed in which for a first target once the scanning has identified the appropriate mirror position to direct the laser beam to the target, an offset is generated to correct the laser transmit mirror""s position. Once an offset or connection factor has been generated, it is used to better aim the laser beam for the next target in the cloud. In one embodiment, the offset is used in a closed loop system so that for this first target a correcting offset is ascertained. Thus for the first target, a global offset is established which is then utilized to correct transmit mirror pointing. This offset is provided to a mapping system so that the transmit mirror is appropriately positioned to narrow the search for the next target.
The ladar then selects a second target and utilizing the global offset projects a laser pulse towards the second target. A second offset is measured because the laser beam is made to execute a search pattern, albeit smaller than the first thanks to the prior correction. The second offset further corrects the transmit mirror pointing, with the amount of searching necessary to identify the exact location of the second target significantly decreased due to the global offset employed in setting the transmit mirror. Once a return is registered from the second target, a further fine tuning offset is entered into the mirror setting database or map.
The ladar designator then searches for a third target and the process is continued. The result is a closed loop system for closing in on a target in which both biasing errors and random pointing errors are significantly reduced to reduce the scanning time spent in the search mode.
Of course it may be that across the entire IR field of view there are different errors for different quadrants or different regions. However, because of the closed loop control of the transmit mirror, errors which are detected are corrected such that the overall scan time for scanning targets by the ladar is significantly reduced.
Thus, regardless of whether or not the IR detector and laser are co-boresighted at the factory and regardless of whatever registration algorithms are utilized in the alignment process at the factory, regardless of the errors due to transport and deployment of the IR detector and ladar system and further regardless of errors induced by thermal considerations and vibration, the subject system provides an active system for reduction of boresight errors in real time.
Thus errors induced in handling of the intercept missile, storage in a silo, and then ultimately launch which might result in boresight misalignments between the IR and the laser are corrected by the subject system without the requirement for uncorrected systems in which laser beam widths need to be expanded to account for boresight errors. It will be appreciated that widening the laser beam drives up the laser power required to illuminate targets, thus impacting negatively on the range of the system.
As can be seen when an incoming missile is sensed by ground based radar and an intercept or kill vehicle is sent up to countermeasure the incoming missile one then needs to avoid boresight ambiguity or boresight errors which leads not only to ambiguities in identifying which target one is looking at, but also where to direct kill vehicles.
The subject system operates on the fact that with the IR detector one can develop a map which has a number of detected targets. One then ascertains their position by knowing where they are within the field of view of the IR detector. One then uses a registration algorithm that indicates where to find the first target and initiates a search pattern with laser beams scanned over an area where the IR detector indicates a target may be. When a reflected laser pulse is detected one therefore has the means to direct a kill vehicle to the target, while at the same time deriving an offset to correct the boresight error. Note that IR targets are reported on the focal plane of the IR device which does not give one knowledge of the position in laser space because of the aforementioned boresight errors.
In short, in the subject system once one finds out where the infrared detector says a target is going to be one then utilizes a computed correction factor to more precisely direct the laser beam to accurately establish target position. Note that there will always be some residual error involved and by going from target to target, when one utilizes the subject system one uses the actual versus predicted location from the IR focal plane to keep refining the measurement. In the subject system the refinement process proceeds very quickly with algorithms used to converge very rapidly on a solution that greatly improves the time line over uncompensated systems.
Note that while the subject invention is described in terms of a ladar which provides angle, angle range 3-D information, the subject system may be used for laser range finders which are already pointed at a known target. As such a laser range finder is a subset of ladars in which only range is detected.
In summary, in a dual mode target designation system involving the use of a passive IR detector for developing rough target locations and a ladar or laser range finder for refining target position, a closed loop system is provided for correcting the boresight error of the laser so that it matches that of the IR detector. In one embodiment this is accomplished by first selecting a target detected by the IR detector, executing a laser scan in which the laser beam is moved in a search pattern until a return from the selected target is detected, developing an error vector between the reported laser target position and the reported IR detector target position and repositioning the line of sight of the laser using the error vector to minimize the co-boresighting error. The result is that the boresight correction resulting from illuminating the first target reduces laser scan time for each subsequent target to permit rapid and accurate target position acquisition. The refined target position may then be used to direct a kill vehicle to the target.